Spark ignition engines operate by igniting a mixture of air and fuel vapors. Gasoline is the most common fuel used today, but this invention is not limited merely to gasoline. The tendency of the fuel to vaporize is important for efficient engine operation and low emission of pollutants. Fuels that do not vaporize readily can cause hard starting, poor drivability during cold operation, excessive byproduct emission and other problems. Conversely, fuel that vaporizes too readily in fuel pumps, fuel lines, carburetors or fuel injectors, etc., can cause decreased liquid flow to the engine and also result in poor engine operation and excess pollutants.
There are several measures of fuel volatility in common use, as for example, the Reid Vapor Pressure (RVP), the distillation Drivability Index (DI), and the Vapor-Liquid Ratio (VLR) or Air Fuel Ratio (A/F). Standard techniques for measuring various fuel properties are provided by the American Society for Testing Materials (ASTM), 100 Barr Harbour Drive, P.O. Box C700, West Conshohocken, Pa. 19428-2959 USA, and are well known in the art. The ASTM also publishes standards for fuels to meet in the USA.
For example, vehicle fuel specification ASTM D-4814 defines vapor pressure and distillation class requirements for six gasoline volatility classes: AA, A, B, C, D, and E. The specification assigns a vapor pressure/distillation profile class each month to each geographical area (state or portion of a state) in the USA based on altitude and the expected ambient temperature range. This data may be conveniently summarized using the distillation Drivability Index defined by the following equation:DI=1.5*(T10)+3.0*(T50)+(T90)  [1]where (T10), (T50) and (T90) are the temperatures in degrees Fahrenheit for 10%, 50% and 90%, respectively, of the evaporated fuel in a distillation test cell. These six volatility classes correspond to gasoline DI values ranging from about 1000 to over 1300. According to a report published by the Chevron Corporation, in 1989 the winter and summer average DI values for gasoline in the USA were about 1030 and about 1127 respectively. It is apparent that fuel properties, as reflected for example in the different DI values, vary substantially during the year, in different geographic locations and with other factors. In order for an engine to run efficiently, smoothly and with improved emissions, it is desirable for the engine control system to adapt to this wide range of fuel properties.
Engine control systems of the prior art have dealt with this variation in fuel properties by using feed-back. For example, measuring the properties of the engine exhaust stream and using this information to adjust the amount of air and injected fuel, engine timing and other parameters so that the A/F ratio more closely approaches stoichiometry. While this works well enough in steady state, it does not work well, for example, during cold starts before the engine and exhaust system have reached normal operating temperatures. During these periods, the engine control system usually relies on stored values for the fuel properties, as for example, one or more stored DI numbers. The stored values must be chosen to reflect the worst-case fuel properties that the engine will likely encounter during any season, altitude, geographical region, ambient temperature, and so forth. Thus, the DI number used by the engine control system, for example, from cold-start through the end of the warm-up period, does not necessarily correspond to the actual properties of the fuel being burned. This can result in poor engine performance and/or excessive hydrocarbon emissions, which are undesirable.
Thus, a need continues to exist for a system for providing the engine control system with information on the volatility properties of the fuel actually on-board the vehicle and about to be burned by the engine. Additional features will become apparent to one skilled in the art based on the foregoing background of the invention, the following detailed description of a preferred embodiment and the appended claims.